Determination of the chloride ion concentration using silver nitrate.

Theory

A titrimetric method based on the formation of a slightly soluble precipitate is called a precipitation titration. The most important precipitation process in titrimetric analysis utilizes silver nitrate as the reagent (Argentimetric process)

Many methods are utilized in determining end points of these reactions, but the most important method, the formation of a colored precipitate will be considered here.

In the titration of a neutral solution of chloride ions with silver nitrate, a small quantity of potassium chromate solution is added to serve as the indicator. At the end point the chromate ions combine with silver ions to form the sparingly soluble brick-red silver chromate.This is a case of fractional precipitation, the two sparingly soluble salts being AgCl (Ksp = 1.2 x 10^-10) and Ag2CrO4 (Ksp = 1.7x10^-12).

AgCl is the less soluble salt and initially chloride concentration is high, hence AgCl will be precipitated. Once the chloride ions are over and with the addition of small excess of silver nitrate solution brick red color silver chromate becomes visible. The titration should be carried out in neutral solution or in very faintly alkaline solution. i.e. within the pH range 6.5-9.

In acid solutions following reaction occurs.

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Consequently the chromate ions concentration is reduced and the solubility product of silver chromate may not be exceeded. In markedly alkaline solution, silver hydroxide (Ksp = 2.3 x 10^-8) might be precipitated.

Procedure

Pipette out 25.00 mL of the chloride solution into a titration flask and add 1mL of the potassium chromate solution. Titrate this solution with 0.1M silver nitrate solution.

I.    Calculate the concentration of the chloride solution
II.   Sketch the titration curve for the above titration.
III. Calculate the concentration of chloride, silver and chromate ions at the equivalence point.